If you you are using mini lights you can buy black out caps. Or you can cut at the mid point. Minis are wire in series of 50 or 35.

For example take a string of 100 follow the wires and you will see three wires then it goes to two and right back to three. Cut at the two wire point and put on a plug and you have shortened your string. Lights of 70 are the same cut mark is between 35 and 36.

I do not use C9's any longer as they burn way too much electric for my home. But C7 5 watt bulbs I use. You can cut that anywhere because it is wired as a normal light. Get some shrink tubing and seal your ends and you are good to go.

Forgive the dumb question but I had to ask. When you get your lights from the supplier how do you make them custom fit to the house? For example when a house has a small part in the roof that is only about 5 feet, how do you light that and not show any lights around it or leading up to it?

You cut the wire between lamps and use quick connectors plugs and sockets to tie them back into the power.